Today, tips are paid because wait staff are paid below minimum wage. This is legal - as tipping is baked into their compensation. It's stupid and anachronistic, but it's not the fault of the staff (though GOOD waiters will often express support for the system because they fear going to fixed wage w/ zero tipping will reduce their income).
I'd much prefer a living wage with optional tips for extraordinary service (which seems to be the norm in much of western Europe). Here in the US, 20% gratuity is pretty much expected. And FWIW, I'll pay 25-30% for good service, and to get less than 20%, a waiter would have to be downright awful.